The law was designed to crack down on sex trafficking. Photograph: David Levene for the Guardian

Men who use brothels are escaping a new law intended to crack down on the coercion of women into prostitution, the Guardian has learned.

Sources at the Metropolitan police and the Crown Prosecution Service said only three men have been cautioned for going with prostitutes who were coerced or threatened into working, since it became a criminal offence in April 2009.

Jacqui Smith, the then home secretary, said the legislation would "turn the spotlight on those who create the demand that leads to trafficking and people being held almost as slaves".

Two men using a brothel in east London were picked up on the day the legislation was introduced and only one other has been cautioned since. The figure emerged as the Association of Chief Police Officers (Acpo) claimed that at least 2,600 prostitutes working in brothels in England and Wales had been trafficked from abroad. Many lived in debt bondage and were strictly controlled through threats of violence to family members.

The figures published by Acpo, relating only to off-street prostitution, suggest that almost one in 10 of an estimated 30,000 prostitutes are trafficked. Half of them come from China with most of the rest from Thailand, other parts of south-east Asia, and eastern Europe. A further 9,200 migrant prostitutes were found to be vulnerable to further trafficking. That group is typically in debt, live outside mainstream society and many have been recruited through abuse of their vulnerability, Acpo said.

The Home Office declined to comment on the apparent failure of previous government policy. Damian Green, the immigration minister, said: "Combating trafficking and looking after its victims is a priority for the new government."

Detectives said the maximum £1,000 fine for "paying for the sexual services of a prostitute subjected to force" means there is little incentive to dedicate resources to it and said it is difficult to prove that prostitutes are being coerced and exploited.

The Acpo figures on trafficked prostitutes represent the best estimate of the scale of prostitution in brothels. Police arrived at the figures after plainclothes officers interviewed foreign sex workers after raiding brothels.

"It provides us with a more sophisticated and nuanced understanding of how migrant women are involved, how they are influenced, controlled, coerced, exploited and trafficked," said Nottinghamshire deputy chief constable Chris Eyre, the Acpo spokesman on migration crime. "The publication of this report represents not the end of the process but the start."

The statistics are considerably lower than previous estimates. A Home Office report in 2003, based on an extrapolation of trafficking in London, estimated that there were 3,812 trafficked prostitutes in England and Wales.

It was described by the authors of the study as "very approximate" but three years later the then home office minister, Vernon Coaker, told a parliamentary committee on human rights: "There are an estimated 4,000 women victims."

In a debate in the Commons in November 2007, Denis MacShane, said that according to home office estimates, "25,000 sex slaves" worked in British massage parlours and brothels.

The Acpo research, Project Acumen – Setting the Record, claims almost 5,000 women from abroad work as sex workers in London in more than 2,000 premises. The majority, 55% of all prostitutes including British prostitutes, came from eastern Europe, while 22.5% came from Asia.

The picture varies dramatically by region. In the south-east region, half of all prostitutes hail from Asia while in Yorkshire and the Humber 68.5% hail from the UK.

The count prompted a fresh row over the measurement of trafficking and prostitution in the UK. Anti-slavery International said the figures represents an underestimation of the problem of trafficking while the English Collective of Prostitutes said the statistics were an over-estimation and claimed that law enforcement agencies use fear of trafficking as a premise for raiding brothels and prosecuting sex workers for lesser offences.

Amnesty International questioned how thorough the "snapshot" is, because only 401 prostitutes from Africa were identified in the research.

The report's authors conceded they may have underestimated the extent of trafficking from China and Nigeria.

In the last six years, there have been 128 convictions for sex trafficking, seven for labour trafficking and three for conspiracy to traffic for sex, according to the Home Office figures.